I assume you mean AIR compressor. That can shut down for any number of reasons and requires trained personnel to diagnose it correctly and safely. The most common cause is just low refrigerant but there are dozens of other possibilities also. I suggest you get it to a qualified A/c tech. A/C is not a DIY job.

I have a 2000 expedition
it has air ride supension and the air compressor will not come on
again Ijumped it an it ran
I replaced the sensor behind the bumper
things still dose not work
and do you know where the relay is for this systom

To test relay, (located below the passenger side headlight). At the 4 wire relay connector, check the Light Blue/Pink wire for battery voltage from fuse #109 50A in the Power Distribution Box under the hood. Check the Light Green/Red wire for a good ground. If OK, back probe the Dark Blue/Yellow wire and jump to ground. The relay should click and battery voltage should be present on the Gray/Red wire to the air suspension compressor. If there is no voltage on the GY/R wire with the DB/YE wire grounded, check connection at the relay. If OK, replace the relay and recheck operation. I have seen the relay connection corrode and in some cases the wire harness/wires rubbed thru near relay.